Reference laboratories

Sometimes typing (Box 29.1) provides clues about the possible country, environmental site or host animal for a specific strain. In addition, as more becomes known about the mechanisms for pathogenicity of pathogens, strain typing (especially the analysis of virulence genes) will be increasingly used to provide information on the ability of a organism to cause disease. For example, Clostridium perfringens is part of the normal faecal flora, but it is also found in the faeces of patients with diarrhoeal disease in whom this bacterium produces an enterotoxin in the enteric tract. Only a small proportion of C. perfringens contain the enterotoxin gene, therefore molecular typing by detection of fragments of this gene by polymerase chain reaction (PCR) is invaluable in distinguishing isolates that have the potential to cause food poisoning from those that do not.